Simple, secure encryption and decryption for Python 2.7 and 3 (now on pypi).

This provides two functions, which encrypt and decrypt data, delegating all the hard work to the pycrypto library (which must also be installed).

The two calls:

from simplecrypt import encrypt, decrypt

ciphertext = encrypt(password, 'my secret')
plaintext = decrypt(password, ciphertext)

A simple Python 3 program:

from binascii import hexlify
from getpass import getpass
from sys import stdin

from simplecrypt import encrypt, decrypt

# read the password from the user (without displaying it)
password = getpass("password: ")

# read the (single line) plaintext we will encrypt
print("message: ")
message = stdin.readline()

# encrypt the plaintext.  we explicitly convert to bytes first (optional)
ciphertext = encrypt(password, message.encode('utf8'))

# the ciphertext plaintext is bytes, so we display it as a hex string
print("ciphertext: %s" % hexlify(ciphertext))

# now decrypt the plaintext (using the same salt and password)
plaintext = decrypt(password, ciphertext)

# the decrypted plaintext is bytes, but we can convert it back to a string
print("plaintext: %s" % plaintext)
print("plaintext as string: %s" % plaintext.decode('utf8'))

Which, when run, produces something like the following (the actual encrypted message will be different each time, as a random salt is used for each message):

password: ******

message:
hello world
ciphertext: b'73630001b1c39575390d5720f2a80e7a06fbddbf2c844d6b8eaf845d4a9e140d46a54c6729e74b0ddeb1cb82dee81691123faf8f41900c5a6c5b755ed8ae195ff2410290bcb8dc2ee3a2126c594b711d'
plaintext: b'hello world\n'
plaintext as string: hello world

Also, it's perhaps worth noting that the overhead (the extra length of the encrypted data, compared to the message) is constant. It looks a lot here, because the message is very small, but for most practical uses should not be an issue.

The algorithms used follow the recommendations at http://www.daemonology.net/blog/2009-06-11-cryptographic-right-answers.html, as far as I can tell:

• The password is expanded to two 256 bit keys using PBKDF2 with a 256 bit random salt (increased from 128 bits in release 3.0.0), SHA256, and 10,000 iterations.

• AES256 CTR mode is used to encrypt the data with one key. The first 64 bits of the salt are used as a message nonce (of half the block size); the incremental part of the counter uses the remaining 64 bits (see section B.2 of http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf).

• An encrypted messages starts with a 4 byte header ("sc" in ASCII followed by either two zero bytes (pre-3.0), or a zero and a one).

• An SHA256 HMAC (of header, salt, and encrypted message) is calculated using the other key.

• The final message consists of the header, salt, encrypted data, and HMAC, concatenated in that order.

• On decryption, the header is checked and the HMAC validated before decryption.

The entire implementation is here.

Discussion and criticism of the design can be found on HN (also), codereview.stackexchange and crypto.stackexchange. Grateful thanks to all commentators (particularly marshray); mistakes remain mine.

Please note that the general design, based on the cryptographic right answers, is intended to give 128 bits of security - any attack would require around 2^128 guesses. This comes from birthday collisions on the 256 bit HMAC and random numbers (since release 3.0). AES256 is used because it provides additional security if, for example, some key bits are revealed through timing attacks (see link above or chapter 7 of Practical Cryptography).

Release 3.0 increases the size of the salt used from 128 to 256 bits. The header has also changed, so data encrypted by previous releases can be detected and decrypted correctly.

Note that data encrypted by release 3.0 onwards cannot be decrypted by earlier releases (instead, an error with a helpful message is generated).

Previous releases were not consistent with the advice on random IDs here. The salt is used both to set the CTR mode offset and to change the key (a random offset alone is not good practice), so must never repeat. Changing from 128 to 256 bits moves the likelihood from "very unlikely" to "not in the age of the universe".

My apologies for not getting this correct in earlier releases.

Release 2.0 should be fully compatible with 1.0 on Python 3 (same API and identical results). However, thanks to d10n it now also supports Python 2.7 (tested with Python 2.7.5, 3.0.1 and 3.3.2).

I (Andrew Cooke) am not sure Python 2.7 support is such a good idea. You should really use something like keyczar. But there seems to be a demand for this, so better the devil you know...

1. The whole idea of encrypting with a password is not so smart these days. If you think you need to do this, try reading about Google's keyczar which instead uses a keystore (unfortunately, at the time of writing, keyczar does not support Python 3, as far as I can tell, but that should change soon).

2. When you call these routines the password is stored in memory as a Python string. This means that malicious code running on the same machine might be able to read the password (or even that the password could be written to swap space on disk). One way to reduce the risk is to have the crypto part of your code run as a separate process that exists for a limited time.

(c) 2012-2014 Andrew Cooke, [email protected]; 2013 d10n, [email protected]. Released into the public domain for any use, but with absolutely no warranty.